Utilize formatted message from AzureEventSourceListener in LogEvent

[christothes]

closes #44028

@lmolkova - If this fix makes sense, there are other files where we should make this fix.

[azure-sdk]

API change check

API changes are not detected in this pull request.

[pharring]

I think the desire in the original code is to defer formatting the message until/unless it's absolutely needed. In particular, you want to avoid (expensive) formatting if the log level would otherwise filter the event entirely.

I would propose an alternative where we add a second (overloaded) constructor to AzureEventSourceListener that takes a simpler Action<EventWrittenArgs>. Callers that don't need the formatted message can use that constructor.

See this: https://github.com/pharring/azure-sdk-for-net/compare/dev/pharring/EventSourceEventFormattingPerf...pharring:azure-sdk-for-net:dev/pharring/AddAzureEventSourceListenerConstructorOverload?expand=1

[christothes]

I think the desire in the original code is to defer formatting the message until/unless it's absolutely needed. In particular, you want to avoid (expensive) formatting if the log level would otherwise filter the event entirely.

I would propose an alternative where we add a second (overloaded) constructor to AzureEventSourceListener that takes a simpler Action<EventWrittenArgs>. Callers that don't need the formatted message can use that constructor.

See this: https://github.com/pharring/azure-sdk-for-net/compare/dev/pharring/EventSourceEventFormattingPerf...pharring:azure-sdk-for-net:dev/pharring/AddAzureEventSourceListenerConstructorOverload?expand=1

As I read it, the formatting doesn't happen until the event is logged either way. Whether it happens higher or lower in the stack seems immaterial. But perhaps I am missing something?

[pharring]

Yes, you are missing something. In this PR, message formatting (likely a string allocation) still happens unconditionally inside AzureEventSourceListener in order to evaluate the arguments to call the LogEvent callback, passing the fully-evaluated formattedMessage to LogEvent. If the log level is set high/low enough that the event will be filtered, then the fully formatted message is never used -- meaning it was a wasted allocation.

[christothes]

Does the EventSource not avoid the eager formatting when it calls IsEnabled before calling WriteEvent?

[pharring]

Does the EventSource not avoid the eager formatting when it calls IsEnabled before calling WriteEvent?

Yes, it does. However, this listener subscribes to events at "Verbose" level (meaning everything) and the AzureEventSourceListener uses calls EnableEvents without passing a keyword mask -- also meaning "all events".